In recent years, large liquid crystal cells have been used in flat panel displays. The liquid crystal cells are frequently constructed by two glass plates joined together with a layer of a liquid crystal material sandwiched thereinbetween. The glass substrates have conductive films coated thereon with at least one of the substrates being transparent. The substrates are connected to a source of power to change the orientation of the liquid crystal material. One of such source of power is a thin film transistor that is used to separately address areas of the liquid crystal cells at very fast rates. The TFT driven liquid crystal cells are useful in active matrix displace such as for television and computer monitors.
As the requirements for resolution of liquid crystal monitors increase, it becomes desirable to address a large number of separate areas of a liquid crystal cell, called pixels. In a modern display panel, more than 100,000 pixels may be present. It requires at least the same number of transistors to be formed on the glass plates so that each pixel can be separately addressed and left in the switched state while other pixels are addressed.
Thin film transistors are frequently made with either a polysilicon material or an amorphous silicon material. For TFT structures that are made of amorphous silicon material, a common structure is the inverted staggered type which can be back channel etched. The performance of a TFT and its manufacturing yield or throughput depend on the structure of the transistor. For instance, the inverted staggered back channel etched TFT can be fabricated with a minimum number of six masks, whereas other types of inverted staggered TFT require a minimum number of nine masks. The specification for a typical inverted staggered back channel etched TFT includes an amorphous silicon having a thickness of 3,000 .ANG., a gate insulator of silicon nitride or silicon oxide, a gate line of Mo--Ta, a signal line of Al/Mo and a storage capacitor. The requirement of a thick amorphous silicon layer in such a TFT device is a drawback for achieving a high yield fabrication process since deposition of amorphous silicon is a very slow process. A major benefit for the amorphous silicon TFT is its low leakage current which enables a pixel to maintain its voltage. On the other hand, an amorphous silicon TFT has the drawback of a low charge current (or on current) which requires an excessive amount of time for a pixel to be charged to its required voltage.
A second major type of TFT structure is made by using a polysilicon material. Polysilicon is more frequently used for displays that are designed in a small size, for instance, up to three inch diagonal for a projection device. At such a small size, it is economical to fabricate the display device on a quartz substrate. Unfortunately, large area display devices cannot be normally made on quartz substrates. The desirable high performance of polysilicon can therefore be realized only if a low temperature process can be developed to enable the use of non-quartz substrates. For instance, in a more recently developed process, large area polysilicon TFT can be manufactured at processing temperatures less than 600.degree. C. In the process, self-aligned transistors can be made by depositing polysilicon, and then gate oxide followed by source/drain regions which are self-aligned with respect to the gate electrode. The device is then completed with a thick oxide layer, an ITO layer and aluminum contacts.
Polysilicon TFTs have the advantage of a high charge current (on current) and the drawback of a high leakage current. It is difficult to maintain the voltage in a pixel until the next charge in a polysilicon TFT due to its high leakage current. Polysilicon also allows the formation of CMOS devices, which cannot possibly be made by amorphous silicon. In a polysilicon TFT, the carrier mobility is about 6 cm.sup.2 /Vs for N-channel devices so that it is suitable for displays as large as 240.times.320 pixel. For the fabrication of larger displays, a higher mobility can be achieved by reducing the trap density around the grain boundaries in a hydrogenation process.
FIG. 1 shows an enlarged, cross-sectional view of a conventional amorphous silicon TFT structure. Amorphous TFT 10 is built on a low cost glass substrate 12. On top of the glass substrate 12, a gate electrode 14 is first deposited of a refractory metal material such as Cr or Ta and then formed. A gate insulating layer 16 is normally formed in an oxidation process. For instance, a high density TaO.sub.x for a Ta gate is frequently formed to reduce defects such as pin holes and to improve yield. Another gate insulator layer 20 is then deposited of either silicon oxide or silicon nitride. An amorphous silicon layer 22 (non-doped) is then deposited with a n.sup.+ doped amorphous silicon layer 24 deposited on top to improve its conductivity. Prior to the deposition of the doped amorphous silicon layer 24, an etch stop 26 is first deposited and formed to avoid damaging the amorphous silicon layer 22 in a subsequent etch process for a contact hole. The doped amorphous silicon layer 24 is formed by first depositing the amorphous silicon layer in a chemical vapor deposition process and then implanting ions in an ion implantation process. Boron ions are normally used to achieve n.sup.+ polarity. A drain region 30 and a source region 32 are then deposited and formed with a pixel electrode layer 34 of generally ITO material deposited and formed on top. The drain region 30 and the source region 32 are normally deposited of a conductive metal layer. A suitable conductive metal can be a bilayer of Cr/Al. The structure is then passivated with a passivation layer 36.
Since both the polysilicon TFT and the amorphous silicon TFT have advantages and drawbacks, it is highly desirable that a different TFT structure can be designed which is cable of maximizing the advantages while minimizing the drawbacks.
It is therefore an object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD that does not have the drawbacks or shortcomings of the conventional polysilicon TFT and amorphous silicon TFT.
It is another object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD that can realize the advantages of both the polysilicon TFT and the amorphous silicon TFT.
It is a further object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD that can be built on a low cost glass substrate normally used for amorphous silicon TFT.
It is another further object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD that can be fabricated at a processing temperature of less than 600.degree. C.
It is still another object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD by transforming partially an amorphous silicon film to a polysilicon film.
It is yet another object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD by converting partially an amorphous silicon film to a polysilicon by laser irradiation.
It is still another further object of the present invention to provide a hybrid polysilicon/amorphous silicon TFT device for switching a LCD by effectively using a n.sup.+ amorphous silicon layer as a mask during a laser annealing process such that part of an amorphous silicon film retains its amorphous nature.
It is yet another further object of the present invention to provide a method for fabricating a hybrid polysilicon/amorphous silicon TFT device for switching a LCD by converting an amorphous silicon partially to polysilicon while retaining the amorphous nature of the film at selected locations.